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Posted: Sun Feb 08, 2004 3:03 pm Post subject: Time lag between casting of beams and slabs
I just want to add two more points to the detailed discussion that has taken place.
1. Only "cold joints" are prohibited. i.e. joints made after the initial setting of first stage concrete and before the final set of first stage concreting.
2. In several of my projects I had to extend slabs which were even 15 to 20 years old. The joints between the old concrete and new concrete have behaved very well. Preparation of the surface of the existing concrete and compaction of the new concrete near the interface are far more important than the time lag. Ofcourse, if the time lag is small the secondary stresses due to differential shrinkage will be much smaller. Also, if the hydration of the existing concrete is still continuing, hydration products will form across the joint, adding to the integrity of the joint. Hence reduction in timelag between old and new concrete will result in a better joint.
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